1. Field of the Invention
The present invention relates to a contact-type image sensor assembly, and, more particularly to a contact-type image sensor assembly for projecting an image of an original document on an image sensor via an imaging lens.
2. Related Background Art
Image sensors for equipment such as facsimile machines and image scanners for use to input images are classified into contraction-type image sensors which contract the image of an original document to be read so as to project it on the sensor and equal-magnification-type image sensors which have an optical system of a 1:1 image focusing type and receiver an image having the same dimension as that of the original document, that is, the same width. An equal-magnification-type image sensor further may have a structure which includes an image focusing lens or may be of a closed contact type which does not include it. Examples of an image sensor which includes an equal magnification imaging lens have been disclosed in Japanese Patent Laid-Open No. 60-230616, Japanese Patent Laid-Open No. 61-25360, Japanese Patent Laid-Open No. 61-25362, U.S. Pat. No. 4,737,654, U.S. Pat. No. 4,724,323, U.S. Pat. No. 4,763,189, U.S. Pat. No. 4,680,644, U.S. Pat. No. 4,920,431, U.S. Pat. No. 4,733,098 and U.S. Pat. No. 4,791,493.
Examples of a closed contact-type image sensor have been disclosed in U.S. Pat. No. 4,924,282, U.S. Pat. No. 4,886,977 and U.S. Pat. No. 4,982,079.
FIG. 1 illustrates an example which uses an equal magnification imaging lens represented by a Selfoc Lens Array (trade name which will be hereinafter called an "SLA") manufactured by Nippon Sheet Glass Co., Ltd. Referring to FIG. 1, reference numeral 1 represents a semiconductor line sensor, 2 represents the aforesaid SLA, 3 represents an LED array for illuminating an original document 5, 4 represents a protection glass on which the original document is placed while being brought into contact with the same, 6 represents a case in which the aforesaid elements are integrally accommodated as a unit and 8 represents a cover for the case 6. Usually, the focal points of the optical system and the other electric characteristics are adjusted in a state where the line sensor 1, the SLA 2, and the LED array 3, and the like, are assembled in the case 6 before delivery to a market as a unit.
Next, the focal point adjustment operation will now be described.
The focal point adjustment operation and the like are performed by vertically moving the SLA 2 by a small quantity. Reference numeral 9 represents a setting screw for securing the SLA 2 to a support wall 6D of the case 6 while pressing the SLA 2 to the support wall 6D after the focal point has been adjusted. The focal point is, as shown in FIG. 2, adjusted by setting a chart (which usually is a white and black stripe chart) 10 for adjusting a lens on the protection glass 4 at a position on which the original document will be placed. Then, the front portion of a focal point adjusting screw 12 is brought into contact with the SLA 2 which is elastically supported by a leaf spring 11 toward a supporting wall 6D, and the SLA 2 is vertically moved by a small quantity while operating the screw 12. Thus, an output obtained from a line sensor (omitted from illustration) is observed by an oscilloscope or the like and the SLA 2 is secured by the setting screw 9 at the moment at which the optimum focal point adjustment is secured. In order to further reliably secure it, an adhesive agent may be used.
Then, the reason why the focal point adjustment must be performed will now be described with reference to FIGS. 3A to 3D.
In general, the optical system, which uses the SLA, must be arranged in such a manner that the distance A from an original document surface 5A to the SLA 2 and the distance B from the SLA 2 to a light receiving surface 1A of the line sensor are the same (that is B=A) (where the distance means an optical distance obtained by dividing the mechanical dimension by the refraction factor of the space medium).
In the process of manufacturing the SLA, the conjugate length Tc which is one of the imaging characteristics is maintained at a guaranteed predetermined value by adjusting the length Z in the direction of an optical axis 31 of the SLA 2 (where the length Z is a mechanical dimension). Therefore, the guaranteed value of the dimension Z of the SLA 2 supplied from an SLA manufacturer is allowed to have a tolerance .DELTA.Z of, for example, about .+-.0.33 mm. However, if the position of the SLA 2 is, as show in FIG. 3B, deviated from the center of the distance between the original document surface 5A and the light receiving surface 1A of the line sensor 1 while maintaining the relative position between them, the focal point is excessively deviated and the SLA cannot be used practically. Accordingly, it has been necessary to perform a focal-point adjustment operation to, as shown in FIG. 3C, always position the SLA 2 at the optical center between the original document surface 5A and the sensor light receiving surface 1A by realizing a state A'=B' by finely moving the SLA 2 in a direction of the optical axis 31 to change the distance A to A' and change the distance B to B'.
In the contact-type line sensor unit assembly thus adjusted, the original document 5 placed on the protection glass 4 is, as shown in FIG. 1, irradiated with light emitted from the LED array 3 and thereby its image is projected on the line sensor 1 via the SLA 2, so that the original document 5 can be accurately read.
However, it takes an excessively long time to complete the focal-point adjustment operation in the process of assembling the conventional contact-type line sensor assembly, causing the overall cost to be increased excessively.
Furthermore, a gap 12 is formed around the SLA 2 because the SLA 2 must be moved along its supporting wall 6D in the direction of the optical axis for the purpose of adjusting the focal point, causing stray light, which has not passed through the SLA 2, to reach the portion around the line sensor 1 through the gap 12. Therefore, there sometimes arises a problem in that the quality of the image read by the line sensor 1 deteriorates.
In addition, if the SLA 2 is tightened strongly by the setting screw 9 in order to secure the SLA 2 to a predetermined position while withstanding shocks given at the time of transportation or during the operation, another problem takes place in that the SLA 2 will be broken by a concentrated load given through the front portion of the setting screw 9 as shown in FIG. 4.